1. Field of the Invention
The present invention relates to an optical head unit for an optical disk apparatus. In this specification, the phrase "optical disk apparatus" refers to a read-only device using an optical disk as well as to an magneto-optical disk apparatus capable of reading and writing data from and in a magneto-optical disk by magnetic field modulation or optical pulse modulation.
2. Description of the Related Art
Conventionally, optical disk apparatus have already been widely used as external mass storage devices for e.g. computers. Nowadays, in preparation for a full-fledged multimedia era to come in the future, a new technology is sought for producing an optical disk apparatus capable of operating with much higher recording density.
FIG. 9 illustrates principal parts of an optical head unit used for a conventional optical disk apparatus. An objective (1) is mounted on an actuator (not shown) for performing track control and/or focus control. The nonillustrated actuator is arranged to bring the objective (1) to a suitable position, so that the laser beam passing through the objective (1) will properly converge on a recording surface (r). As is shown, the recording surface (r) is attached to the upper side of a transparent platter of an optical disk (d). An element designated by reference character (h) is a magnetic head.
For increasing the recording bit density of the optical disk, it is possible to take the following measures: (a) utilization of a laser semiconductor as a light source that is arranged to emit a laser beam having a shorter wavelength; (b) utilization of an objective having a greater numerical number (NA); and (c) utilization of magnetic field modulation.
At present, the above option (a) is not practical since a laser semiconductor having a shorter wavelength (i.e., blue laser semiconductor) has several problems to be solved in respect to costs, output efficiency, thermal stability and so on.
Regarding the option (b), when use is made of only one objective for providing a relatively high NA, the radius of curvature of the objective should be small. However, such a lens is difficult to manufacture. Further, as the NA becomes greater, there will be an unfavorable increase in the coma-aberration (which is proportional to the third power of the NA) due to, for instance, tilt or eccentricity between the incidence plane and the exit plane of the objective, or due to tilt or eccentricity of the objective itself, or due to tilt of the optical disk. Further, spherical aberration (which is proportional to the fourth power of the NA) will also increase due to uneven thickness of the optical disk. With those aberrations present, it will be almost impossible to properly write and read data in and from the optical disk.
JP-A-8-221790 discloses an optical pickup apparatus which is proposed for dealing with the problems stated above. Referring to FIG. 10 of the accompanying drawings, the disclosed apparatus is provided with an object lens assembly including a first objective (11) and a second objective (12) each having a small NA. The second objective (12) is supported by a lens-barrel (m) of a serve-control actuator (a), while the first objective (11) is attached to a slider (s) which is supported by the lens-barrel (m) via an elastic member (b). In operation, the slider (s) will either be held in sliding contact with the bottom surface of a disk (d) or be caused to float away from the bottom surface of the disk (d) via an air layer.
According to the arrangements illustrated in FIG. 10, use is made of a combination of two objectives, i.e., the first and second objectives (11, 12) so that the overall NA of the object lens assembly is increased. As a result, it is possible to provide a higher recording density, while also overcoming the problems caused by using a single objective having a small radius of curvature. Further, the slider (s) can be held close to the bottom surface of the rotating disk (d) under action of the elastic member (b). In this manner, the coma-aberration due to the tilt of the disk (d) is prevented, while the wave aberration of the lens assembly being also eliminated. Thus, the writing and reading of data in relation to the disk (d) can be performed with a high bit density.
However, in the conventional optical pickup apparatus shown in FIG. 10, the slider (s) carrying the first objective (11) is attached to the mirror-barrel (m) of the actuator. Such an arrangement will give rise to the following disadvantages.
In general, when use is made of a two-dimensional actuator movable in a focus control direction (vertical direction) and a track control direction (horizontal direction), the actuator may be carried by a carriage (movable in a radial direction of an optical disk) via a support spring. Further, the actuator may be provided with a servomechanism having a focus coil and a track coil which are positioned in the magnetic field generated by a certain circuit mounted on the carriage. In such an arrangement, based on detected signals obtained from laser beams reflected on the recording surface of the optical disk, electric currents will be supplied to the track coil and/or the focus coil. Accordingly, the actuator will be moved in the track control direction and/or in the focus control direction for performing track control and/or focus control. In order to enable fast response for those control operations, it is preferable that the inertial mass of the actuator is rendered as small as possible, and that the rigidity of the support spring attached to the carriage is minimized.
However, according to the conventional optical pickup apparatus shown in FIG. 10, the actuator (a) carries the slider (s) via the elastic member (b), which unfavorably puts an additional weight onto the actuator (a). Another disadvantage is that a very complicated mass-spring system is formed between the carriage and the slider (s) when the slider (s) is elastically urged onto the disk (d). Regarding the first-mentioned disadvantage (unfavorable increase in weight), when the actuator (a) cannot be moved with sufficiently fast response, the floating and tilting extents of the slider (s) will unfavorably vary, which may produce a greater wave aberration. When this happens, high-density data writing and reading operations will become difficult to perform. At the same time, the driving performance of the actuator (a) will become unstable when even a slight error occurs in mounting the slider (s). Regarding the second-mentioned disadvantage (formation of a complicated mass-spring system), specifically, the carriage supports the actuator (a) via the support spring, the actuator in turn supports the slider (s) via the elastic member (b), and the slider (s) is associated in motion with the optical disk (d) via an air film when the disk is rotated. In such an arrangement, when the rotating disk (d) causes vibrations in the slider (s) due to the tilt and/or uneven thickness of the disk (d), the unsteady movement of the slider may lead to the resonance of the above mass-spring system. When this happens, the extent of the floating and/or tilting of the slider (s) in relation to the disk (d) tend to vary, and the wave aberration will become greater. In addition, the resonance mentioned above will render the focus control and the track control unreliable, whereby proper writing and reading operations with respect to the disk (s) will become difficult.